A magnetic resonance device serves for recording or generating images of an examination object that are acquired by the signals resulting from a radio frequency excitation and in which the image is subsequently determined or reconstructed. To this end, a basic field magnet is used to generate a basic field that is as homogeneous as possible and has a homogeneity volume of defined homogeneity. Superposed on the field for imaging purposes is a gradient field generated by way of a gradient coil and having field components in the x-, y- and z-directions. Finally, a radio frequency coil produces an RF pulse for the spin excitation that leads to the signal generation. The design and mode of operation of a magnetic resonance device are adequately known to the person skilled in the art and require no further explanation.
The recording of a magnetic resonance image or a tomogram is preferably performed at the center of the approximately spherical homogeneity volume, the so-called isocenter. In the context of a so-called “isocentering” concept recently introduced, the slice set to be measured is positioned at the isocenter by automatic table displacement for each protocol used for a measurement, that is to say a signal recording for imaging. That is to say, for each slice to be recorded the patient is readjusted as necessary such that the body region in which the slice lies is positioned at the isocenter. This ensures the best possible basic field homogeneity and gradient linearity, and thus image quality, for the volume to be imaged.
In order to avoid systematic errors during the slice planning, that is to say the definition of the slices to be recorded, the slice planning may be done in the context of the isocentering concept exclusively on distortion-corrected images. Such images are corrected by geometric distortions that result from gradient nonlinearities. When constructing a magnetic resonance image using one or more algorithms starting from the recorded measurement signals, it is firstly assumed that there is an ideal linear gradient field.
However, real gradient fields deviate from this idealized linear profile and have nonlinear components. This additional nonlinearity has the consequence that a signal measured at a first real location falsely appears at a second, other location after the reconstruction. Via the so-called distortion correction, these errors are corrected on the basis of knowledge of the spatial nonlinearity of the gradient fields with the aid of the algorithm or algorithms used, which have corresponding correction sections. Since these distortion-corrected images consequently exhibit images that are geometrically or anatomically correct, the aim is to apply the distortion correction to all measured images.
Since specific applications such as, for example, spectroscopy, necessarily dictate the use of undistorted images in order to ensure an absolute positioning accuracy of protocol planning and of imaging evaluation, it is not the processed, distortion-corrected images or data records that are filed, but the originally measured data records that exhibit the distortions owing to the nonlinearities. Storing both the originally recorded image data records, that is to say the distorted 2D or 3D reconstruction images, as well as the distortion-corrected reconstruction images in the image database is not an option, since this doubles the image data volume to be stored.
Consequently, evaluating images in the context of slice planning, which in by far most cases is performed with the aid of the distortion-corrected images, always requires these images to be recalculated from the original measured data. Thus, in a normal operation there is consequently a need for considerable and time consuming computer performance resulting from the requirement that the undistorted image data be on hand for a few applications.